This invention relates generally to examination of nuclear reactors, and more particularly, to the examination of a top weld of a core shroud of a boiling water nuclear reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide, sometimes referred to as a grid is spaced above a core plate within the RPV. A core shroud, or shroud, surrounds the core plate and is supported by a shroud support structure. The core shroud is a reactor coolant flow partition and structural support for the core components. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. A removable shroud head is coupled to a shroud head flange at the top of the shroud.
The shroud, due to its large size, is formed by welding a plurality of stainless steel cylindrical sections together. Specifically, respective ends of adjacent shroud sections are joined with a circumferential weld. During operation of the reactor, the circumferential weld joints may experience intergranular stress corrosion cracking (IGSCC) and irradiation-assisted stress corrosion cracking (IASCC) in weld heat affected zones which can diminish the structural integrity of the shroud. In particular, lateral seismic/dynamic loading could cause relative displacements at cracked weld locations, which could produce large core flow leakage and misalignment of the core that could prevent control rod insertion and a safe shutdown.
Known methods of inspecting the circumferential shroud welds for IGSCC and IASCC utilize ultrasonic probes positioned on the shroud outer surface at the weld joint. A series of scans are performed while projecting the ultrasonic beam through the weld from the outer side of the shroud to the inner side of the shroud. Some methods position the probe on the inner surface of the shroud and project the ultrasonic beam from the inner surface of the shroud to the outer surface of the shroud. The weld between the shroud head flange and the upper shroud section, sometimes referred to as an H1 weld, is very difficult to access for inspection because of the plurality of shroud head locking lugs located around the outer surface of the shroud head flange which limits access to the weld from the outer surface of the shroud. Typically, less than 80% of the weld area can be examined. Additionally, because the shroud head flange extends radially inward, a probe cannot easily be placed against the weld between the flange and the upper shroud section on the inner surface of the shroud. Placing probes below the weld under the flange ledge and performing scans of the weld and upper heat affected zone by directing the ultasonic sound beam through the weld from the lower side has produced unreliable detection readings.
It would be desirable to provide a method of inspecting the H1 weld between the shroud head flange and the upper shroud section that is reliable and that reliably examines greater than 80% of the weld circumference.